Porcelain post insulators for high-voltage lines A line post insulator is a steel insulator consisting of one or more insulators permanently glued on a metal base, sometimes with a cap, and can be double mounted on a metal base. The head bolt or one or several bolts are rigidly mounted on the supporting structure. It is used for insulating and supporting wires in AC power lines with a nominal voltage higher than 1000V, a frequency not higher than 100Hz, and an altitude not higher than 1000m. between 40°C. Line (solid) post insulators can replace pin insulators.